This invention relates to trailer hitches and particularly to improved controlling systems connected to the hitches, or fifth-wheel mountings, for operating automatically the brakes of trailers.
In U.S. Pat. No. 3,397,899 issued to Theodore F. Thompson on Aug. 20, 1968, a fifth-wheeled hitch with an improved controlling system with stabilizing means is described. Two brackets are mounted on opposite sides of a frame at the rear of a tractor to secure a fifth wheel for supporting the front end of a trailer. The fifth wheel is mounted to the pair of brackets through a pair of upright supporting links that permit the fifth wheel to move about 1/2 inch (1.27 cm) to 1 inch (2.54 cm) fore and aft with respect to the frame of the tractor. A control for the brakes of the trailer that is being pulled is coupled to the upright links and is operated as force applied from the fifth wheel causes the links to move through the short distance.
While a tractor is pulling a trailer, the fifth wheel is in a rearward position to actuate the braking control for maintaining the brakes of the trailer released. Should the momentum of the trailer tend to cause the trailer to overrun the tractor, the fifth wheel is pushed forwardly with respect to the frame of the tractor to apply brakes in sufficient amount to slow the traler and to return the fifth wheel to a rearward position.
To provide stability, an indexing or toggle action for the upright supports is required. When the fifth wheel has moved to either a forward or a rearward position, a substantial change of force in the opposite direction must be required to return the fifth wheel to its former position. In the embodiment described in the patent to which reference has been made above, each of the upright links has a pivot at the bottom and has a stop arrangement that permits the upper portion of the link to move a short distance fore and aft to travel through the position at which the load or downward force is supported directly over the pivot. Through this construction, the front end of the trailer is at its highest point with respect to the frame of the tractor when the fifth wheel is at an intermediate point with respect to the fore-and-aft movement of the fifth wheel and gradually assumes a slightly lower position with respect to the frame as the fifth wheel moves past the intermediate point. The amount of indexing is therefore directly proportional to the load on the trailer.
When a trailer is heavily loaded, this type of hitch has not functioned satisfactorily. When a heavily loaded trailer with its tractor is stopped, the fifth wheel is in a forward position and the brakes of the trailer are applied. Before the trailer can be started forward again, either the brakes must be released by a special manual control or the tractor must provide sufficient traction to raise the load a small amount required to move the fifth wheel in a rearward position. Although the vertical distance through which the trailer is to be moved is very small, the amount of starting traction to be provided by the tractor is undesirably great.